6.3.2.2 1-F ALIGNMENT
Following completion of the trap and tuner
link adjustments there are essentially two
methods of completing the balance of the i-f
alignment. The first of these methods is to
use modulated marker frequencies, taken one
at a time, to prealign all tuned circuits in the
i-f strip (This procedure is sometimes called
AM alignment.). In other words, each tuned
circuit is preset to a frequency specified by
the manufacturer. Conventional sweep align-
ment is then used for final adjustment of the i-f
circuits using reference markers (post injec-
tion). This method of pre-alignment of all
tuned circuits in the i-f strip is superior be-
cause once all the coils have been pre-aligned
to specific frequencies the final alignment
using sweep frequencies is greatly simplified
because all that is required is a slight touch-
up of all coils which have been previously
spot-aligned. Another advantage of the pre-
alignment method is that the operator is given
the tuned frequency of each circuit in the i-f
strip and there is no possibility of confusion
on the part of the operator in deciding which
portion of the i-f response curve is affected by
a particular tuning adjustment. The alternate
method is that once having aligned the traps
and the tuner link, the balance of the i-f is
aligned using conventional sweep alignment
and adjusting the balance of the tuned cir-
cuits as required to obtain the desired overall
response curve.
An extensive review of the alignment proce-
dures of many older television sets as well
as procedures for new sets currently appear-
ing on the market indicate that the following
frequency and gain distributions apply. First
the output tuned circuit of the first i-f amplifier
stage is tuned toward the upper end of the
i-f passband, and is used in sweep alignment
to locate the 45.75 (pix carrier) marker on the
i-f response curve. The output tuned circuit
of the second i-f amplifier is tuned to the
lower end of the i-f bandpass and adjustment
of this transformer affects this end of the i-f
response curve with respect to the 42.17 MHz
(chroma carrier) marker.
The output section of the third i-f amplifier
usually consists of a double tuned coil as-
sembly. One of these coils, usually the top
coil, is used for trapping the sound carrier
(41.25 MHz) and the lower coil or bottom slug,
as the adjustment is usually called, is tuned
to the center of the i-f bandpass or to approx-
imately 44 MHz. Tuning of this coil affects the
amount of tilt obtained in the i-f response
curve.
6.3.2.3
CHROMA ALIGNMENT
54
In alignment of the chroma portions of the
receiver several methods are used. One meth-
od is to pre-align the bandpass amplifier out-
put using direct video sweep applied at the
grid (or base, if solid state) of the bandpass
amplifier. The output of the bandpass ampli-
fier is observed using a demodulator circuit.
Some manuals show a schematic diagram of
a chroma demodulator which is to be con-
structed and used in observing the chroma
,,response curve. The PR-151 Demodulator
Probe which is supplied with the Model 415
is capable of performing this function and
should be used.
Once a bandpass amplifier circuit has been
aligned, video sweep modulation or an equiv-
alent sweep signal is applied at the mixer or
at the antenna and the overall response of the
chroma circuit is observed at the output of
the bandpass amplifier.
Also included in most alignment procedures
for the chroma circuitry is the adjustment of
the chroma takeoff coil. This coil is located
at the input to the first chroma amplifier or,
if only one chroma amplifier stage is used, it
is located at the input to the bandpass ampli-
fier. Alignment of this takeoff coil is done
either in conjunction with alignment of the
chroma bandpass transformer or as a sepa-
rate alignment step.
6.3.2.4
RECORDING ALIGNMENT REFERENCE
INFORMATION ,
The preceding comments on i-f alignment are
intended as a guide to the user in analyzing
new or unfamiliar alignment procedures. As
the user accumulates alignment experience
on the television receivers of various manu-
facturers he will recognize that many similar-
ities exist among the various procedures.
Certain marker and reference frequencies are
continually repeated through alignment pro-
cedures. In addition, the user should analyze
each alignment procedure with reference to
the circuit schematic. He should also make
brief notes on new sets which he encounters.
These notes at the minimum should identify
the various tuned circuits in the i-f strip and
also note which markers are observed while
adjusting the various tuned circuits. Bias
points and injection and monitoring test points
should also be noted. Pitfalls should be
logged to be avoided
in the future. If this
procedure is maintained over a period of time
the user will soon begin to recognize the sim-
ilarities in i-f design with respect to frequency
and gain distribution.
In addition, he will ob-
serve that all current models of a particular
manufacturer have the same alignment pro-
cedure, or, if they vary,
it is only with minor
differences. Faithful adherence to the
type of
note taking outlined here will produce a sur-
prising acceleration in the alignment process.
I
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